1. Field of the Invention
During the latest decade there has been an increased focus on the purification of waste water from urban and industrial activity prior to leading it back to nature.
Numerous private housings are not connection to public sewerage systems and their waste water is therefore not treated in a public purifying plant. Instead these housings rely on their individual solutions on the purification issue.
A common solution is in form of a septic tank in which an anaerobic fermentation process at low temperatures purifies the waste. This process however takes a significant time and the outflow is often discharged to some kind of percolation through the ground, e.g. a seepage pit, or through a drainage tube o the nearest lake or stream.
In Denmark alone over 300.000 housings only rely on a septic tank as waste water treatment.
Resent demands from the authorities force these housings to provide improved purification of their waste water. Among others there are focus on organic compounds, phosphorous compounds and nitrogen containing compounds.
The phosphorous compounds are often removed by an oxidative precipitation in which chemicals are added to the waste water and oxidize the phosphorous compounds to phosphates which are precipitated as sparingly soluble salts. The phosphorous compounds are normally either precipitated in a separate tank or in the septic tank or optionally in a compartment in the septic tank as disclosed in JP 1997 004 1289.
2. Description of the Related Art
Prior plants for purifying the waste water from one or more of the above mentioned compounds are described in the literature.
US 2003/0062321 A1 describes a plant in which water to be purified is supplied to a first anaerobic filter bed and from there to a second anaerobic filter bed. During these steps organic matter is decomposed anaerobically and nitrate being an oxygen source is converted into free nitrogen. From the second anaerobic filter bed the waste water is led to an aerobic filter bed in which organic nitrogen is converted biologically into nitrate. After precipitation water is then lead to disinfection and electrolytic de-nitrification or to electrolytic phosphorous removal. This process has the disadvantage that phosphorous compounds are not removed prior to introduction into the microbial zones, where it can influence the growth and purifying ability of the bacteria. Further the nitrification in the aerobic filter bed, being autotrophic, might be hampered by competition from faster growing heterotrophic bacteria thus resulting in incomplete conversion of nitrogen containing compounds. Still further there is no description on how to handle a possible problem with H2S.
JP 10 235104 describes a phosphorous removing device which can be applied to different sections of a waste water purification plant. The phosphorous removing device may be provided in any section of a water purifying plant having two anaerobic sections and one aerobic section. In the illustrated case the phosphorous remover device is provided in the second anaerobic section. This construction has the disadvantage that excessive amount of phosphorous removing chemical has to be used, that the autotrophic bacteria do not have optimal growing conditions and that optionally present H2S is not removed.
U.S. Pat. No. 6,210,578 B1 discloses a water treatment plant to be used in series in connection with a septic pit. The water treatment plant has a plurality of aerated compartments having aerobic bacteria and two anaerobic compartments. Prior to the anaerobic compartments there a sedimentation compartment in which dense material settles. In this plant water is recycled from the anaerobic compartment to the sedimentation compartment. This provides improved flow through the system and allows for de-nitrification in the anaerobic compartments. Further it dilutes toxic compound to levels acceptable for the bacteria. This plant has the disadvantage that although it dilutes toxic compounds it does not prevent them from being generated. In case of holidays and a break of the supply of water to be treated a significant amount of H2S is generated in the septic pit and afterwards when the supply is resumed, and a flow through the system is generated, too much H2S to be sufficiently diluted enters the treatment plant. Thus the bacteria are killed and insufficient water treatment is provided resulting in outlet of unpurified water. Further this plant does not have an optimal phosphorous removal.
WO 03/02650 discloses a water treatment plant having two aerated sections and a stripping compartment prior to the aerated sections. The stripping compartment removes H2S from the waste water. This plant has some of the drawbacks described above. It does not inhibit H2S from being generated and it does not have optimal phosphorous removal.
The object of this invention is to provide a method and a plant which has improved phosphorous removal, which has improved start up performance after longer breaks and which limits the generation of H2S.